Implantable electrical devices, such as pacemakers and intra-cardioverter defibrillators have become increasingly sophisticated over the last decade. These types of implantable electrical devices typically have a control unit and one or more leads which are implanted in or around the heart. The control unit induces the one or more leads to deliver a therapeutic electrical stimulus to the heart. Generally, the control unit is positioned within a casing that is configured to be positioned within the body of the patient. One typical place for implantation of the casing is under the patient's pectoral muscle, i.e., a pectoral implant.
The control units generally incorporate a power supply, a processor and several sensors. The sensors provide the processor with pertinent information to allow the processor to induce the leads to apply appropriate therapeutic stimuli to the heart. These sensors can include activity sensors which provide the processor with an indication of the activity level of the patient and body position sensors that provide the processor with an indication of the orientation of the patient. Further, it is common to have a sensor positioned adjacent the wall of the heart to provide the processor with signals indicative of the activity of the heart. Typically, an intracardiac electrogram (IEGM) is provided by this sensor to the processor. All of this information is then used by the processor to determine whether to apply an electrical therapeutic stimulus to the heart and is also used to determine the configuration of the applied stimulus.
One example of a sophisticated implantable electrical device is a demand-type pacemaker which senses the intrinsic activity of the heart and is then provides an electrical therapeutic stimulus to the heart only when it senses an absence of an appropriate intrinsic activity of the heart. This type of pacemaker is therefore providing the stimulus to the heart only on an as-needed basis thereby minimizing the drain on the power supply and limiting the disruption of the heart's natural function. This is in contrast to a pacemaker which provides the stimulus at the heart at a fixed rate regardless of the intrinsic activity of the heart.
As the implantable electrical devices have become increasingly more sophisticated, these devices are capable of providing different therapy to the heart based upon the sensed condition. Further, these devices are also capable of storing data indicative of the sensed activity of the heart during a particular interval of time for future downloading to an external monitor. This permits subsequent evaluation of the downloaded data by the treating physician.
With the increased capability of the implantable electrical devices and, in particular, the variety of possible functions and modes of operation of the device, there has been a desire to be able to change the device's mode of operation or have it initiate new functions simply and easily. Sophisticated implantable electrical devices include telemetry circuitry which allows for communication between the implanted device and an externally monitoring system. Typically, the patient goes to a physician's office which is equipped with the external monitoring system where the monitoring system communicates with the processor, generally through the use of RF signals. This allows the physician to monitor the performance of the implantable electrical device and also allows the physician to reprogram the processor to change various performance parameters of the device.
While the telemetry circuitry greatly enhances the ability of the treating physician to program the implantable electrical device for the specific symptoms being exhibited by the patient, it does require that the patient travel to the physician's office to have the processor reprogrammed. However, there are circumstances where it is desirable to be able to change a performance parameter of the device without requiring the use of a complicated telemetry circuit.
For example, the patient may detect an abnormality in the function of either their heart or the implanted electrical device. These conditions could be either simply corrected or recorded for subsequent evaluation by the initiation of a specific pre-programmed activity performed by the device. Hence, there has been a need for some type of device that would allow the patient or physician to initiate a preprogrammed activity in the device such as a new mode of operation or a new function in a simpler manner than using a telemetry system.
To address this need, implantable electrical devices are often equipped with a reed switch which closes upon the application of an external magnetic field. Reed switches are described in some detail in an article entitled "The Magnetic Reed Switch in Pacemaker Mode Switching," by Jack Driller and Victor Parsonnet, M.D., published in Medical Instrumentation, Vol. 8 (1974) pp. 316-321. Specifically, the reed switch has two metal strips of a deformable magnetic material that are positioned within a glass capsule. The strips are mounted so that in the absence of an external magnetic field, the strips do not touch one another. However, when an external magnetic field is applied to the strips, the two strips bend and make contact with one another thereby closing a circuit. The switch is preferably electrically connected to the processor so that the closure of the switch in response to the applied external magnetic field is sensed by the processor thereby allowing the processor to initiate a preprogrammed activity such as a preprogrammed mode of operation or a preprogrammed function.
While reed switches enable the patient or a doctor to quickly and efficiently alter the performance of the implanted device, reed switches have several disadvantages. One disadvantage is that the reed switch may be accidentally triggered. Specifically, the patient may inadvertently be in the presence of a strong magnetic field which could trigger the reed switch. This is the result of reed switches being incapable of distinguishing between magnetic fields of different magnitudes. The strong magnetic field may be the result of the patient being around industrial equipment or undergoing a medical procedure such as magnetic resonance imaging (MRI). The inadvertent closing of the reed switch in such a circumstance may result in the implantable electrical device switching to an undesired mode of operation or initiating an undesired function.
Further, a reed switch is a comparatively large component. The processor and control circuitry for pacemaker and intra-cardioverter defibrillator (ICD) applications is preferably as small as possible so that the invasiveness of the implanted device is reduced. However, the reed switch is often large and consumes a significant amount of the limited space within the casing containing the control circuitry.
Yet another difficulty with reed switches is that these switches can be unreliable in operation. As the function of the switch relies upon a mechanical occurrence, i.e., the mechanical movement of the two reeds in response to the applied external magnetic field, variations in the material comprising the reeds may result in variations of performance of the reed switch. Hence, application of external magnetic fields in some circumstances may not result in actuation of the reed switch in the desired fashion. Further, repeated actuation of the reed switch may result in fatigue in one of the reeds to the point where the reed may become unreliable. Additionally, the reeds may occasionally remain stuck together after the application of the external magnetic field. In this circumstance, the reed switch may be continuously inducing the processor to perform a specific function that is not needed. This can also result in the reed switch no longer being capable of sensing future applications of the applied magnetic field.
From the foregoing, it should be apparent that there is a need for an improved sensing device for use in an implantable electrical device which is capable of detecting the presence of an externally applied magnetic field. In particular, the improved sensing device should be less susceptible to inadvertently applied magnetic fields and should be more reliable. To this end, there is a need for a non-mechanically activated magnetic field sensor that is small in size and can be implanted within the body of a patient so as to provide the implantable electrical device's processor with a signal indicative of the application of a specific preselected externally applied magnetic field.